chore: add Platform API contract review skill

[abhishek1234321]

Summary

Adds a repo-local Codex skill for reviewing Sage Expense Management Platform API contract changes across:

• fyle-platform-api runtime schemas, handlers, models, database views, migrations, and fixtures
• fyle-platform-docs/src source OpenAPI definitions and generated reference bundles
• fyle-platform-types synced specs, generated TypeScript, and version classification
• internal and external API/SDK consumers

The skill is read-only by default and returns proposed, evidence-backed review comments unless publishing is explicitly authorized.

Skill structure

• SKILL.md: concise review workflow and publication guardrails
• references/contract-chain.md: verified repository relationships, commands, and evidence map
• references/review-examples.md: request/response requiredness, OpenAPI 3.0 nullability, and generated SDK compatibility examples
• agents/openai.yaml: generated Codex UI metadata

Validation

• Initialized and generated metadata with the official skill-creator tooling.
• quick_validate.py: Skill is valid!
• Confirmed only name and description are present in skill frontmatter.
• Confirmed all referenced files exist, no TODOs/placeholders remain, and SKILL.md is under 500 lines.
• Whitespace and staged-diff checks passed.
• Independently forward-tested against fylein/fyle-platform-docs#806 in read-only mode.
• The forward-test identified request-versus-response requiredness, OpenAPI 3.0 $ref nullability, personal-card runtime guarantees, generated enum/source compatibility, and duplicate-comment avoidance.
• Isolated generated-type validation passed 9 tests with 4 intentional skips.
• No GitHub review comments were posted during validation.

ClickUp

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